1. Field of the Invention
The present invention relates generally to converting electrical signals to optical signals, and more particularly to adaptively equalizing (i.e., nulling) any intermodulation distortion effects resulting from such conversion of electrical signals to optical signals.
2. Related Art
Data transmission over an optical communication medium, such as a fiber optic cable, is typically achieved by first converting a data signal to a modulating signal. Both the data signal and the modulating signal are analog RF (radio frequency) modulated electrical signals. The modulating signal is applied to a laser to produce a modulated optical signal such that the optical signal is an optical representation of the electrical data signal. In other words, the optical signal is modulated using the modulating signal such that the optical signal is embedded with the information contained in the data signal. This optical signal is then transmitted to appropriate destinations via the optical communication medium.
However, the modulation response of a laser when an analog or digital RF modulating signal is applied is not linear. Such nonlinear modulation response characteristics result in the generation of intermodulation distortion on the transmitted signal. The intermodulation distortion degrades the quality and fidelity of the optical signal to be transmitted and also limits the span of optical fiber over which the signal may be transmitted.
A prior solution to this problem involves the use of a tuned hybrid circuit to compensate for a laser's non-linearity modulation response pattern. This tuned hybrid circuit generates a first signal and a second signal, where the first signal has a first frequency, a first phase, and a first amplitude, and the second signal has a second frequency, a second phase, and a second amplitude.
A human operator analyzes the intermodulation distortion products and then implements the tuned hybrid circuit using appropriate passive components (such as capacitors and resistors) so as to set the values of the first and second frequencies, phases, and amplitudes. Ideally, the first and second frequencies, phases, and amplitudes are set by the human operator so that the first and second signals are equal in amplitude but opposite in phase to the intermodulation distortion products. The first and second signals are used to produce the electrical modulating signal such that the optical signal produced by the laser in accordance with the electrical modulating signal does not contain any intermodulation distortion products.
This prior solution is flawed since it is subject to human error. The intermodulation distortion products are canceled only if the human operator accurately determines and sets the first and second frequencies, phases, and amplitudes.
This prior solution is also flawed because it does not automatically compensate for changes in the phase and/or amplitude of the intermodulation distortion products. As will be appreciated by persons skilled in the art, the modulation response pattern of a laser drifts over time. Such drifts result in phase and/or amplitude changes in the intermodulation distortion products generated by the laser. Since it must be manually tuned, the conventional tuned hybrid circuit of the prior solution is not capable of automatically adjusting to changes in the phase and/or amplitude of the intermodulation distortion products.
Thus, what is required is an adaptive equalizer which, without any human intervention, automatically nulls intermodulation distortion products generated as a result of a laser's non-linear modulation response pattern, and which automatically compensates for phase and/or amplitude changes in the intermodulation distortion products caused by drifting of the laser's modulation response pattern.